Sensors are key feedback devices on many electromechanical systems. There is a wide variety of sensors available and new sensor technologies are continuously being developed. One of the most common position sensors utilized to measure the moving parts within a mechanical system is the optical encoder. An optical encoder is a closed-feedback device that converts motion or positional information into digital signals that may be utilized by a motor control system.
Optical encoders produce a digital output based on an encoded media (such as a codewheel or codestrip) that passes either through or by the optical encoder. In general, this encoded media is encoded with alternating light and dark regions (or slots) on the surface of the encoded media. When operated in conjunction with this encoded media, the optical encoder translates rotary or linear motion into a two-channel digital output.
Typically, optical encoders are either linear optical encoders or rotational optical encoders. Linear optical encoders may determine the velocity, acceleration and position of a read-head relative to an encoded media (such as a linear codestrip) utilizing a linear scale, while rotational optical encoders may determine the tangential velocity, acceleration and angular position of a read-head relative to an encoded media (such as a codewheel) utilizing a circular scale. Generally, both linear and rotational optical encoders may be implemented either as transmissive, reflective or imaging optical encoders.
In FIG. 1, a side cross-sectional view of a typical transmissive optical encoder 100 in combination with an encoded media (such as codestrip or codewheel) 102 is shown. The optical encoder 100 may include a read-head 104, where the read-head 104 may include an emitter module 106, and a detector module 108. The read-head 104 and the encoded media 102 may move freely relative to each other in either a linear or rotational manner based on whether the transmissive optical encoder 100 is either a linear or rotational optical encoder.
The emitter module 106 and the detector module 108 may include optics capable of emitting and detecting, respectively, optical radiation 110 from the emitter module 106 to the detector module 108. The optical radiation 110 may be visible, infrared, and/or ultraviolet light radiation. The emitting module 106 may include a light source (not shown) such as a diode, a light emitting diode (“LED”), photo-cathode, and/or a light bulb, and the detector module 108 may include an array of photo-detectors (not shown) such as photo-diodes, photo-cathodes, and/or photo-multipliers.
In FIG. 2, a side cross-sectional view of a typical reflective optical encoder 200 in combination with an encoded media 202 is shown. The reflective optical encoder 200 may include a read-head 204, where the read-head 204 may include an emitter module 206, and a detector module 208. Similar to FIG. 1, the read-head 204 and the encoded media 202 may move freely relative to each other in either a linear or rotational manner based on whether the reflective optical encoder 200 is either a linear or rotational optical encoder.
The emitter module 206 and the detector module 208 may include optics capable of emitting and detecting, respectively, optical radiation from the emitter module 206 to the detector module 208. The optical radiation may include emitted optical radiation 210, which is emitted by the emitter module 206 on to the encoded media 202, and reflected optical radiation 212, which is reflected to the detector module 208 by the encoded media 202.
It is appreciated by those skilled in the art that the optical radiation again may be visible, infrared, and/or ultraviolet light radiation. The emitting module 206 may include a light source (not shown) such as a diode, a LED, photo-cathode, and/or a light bulb, and the detector module 208 may include an array of photo-detectors (not shown) such as photo-diodes, photo-cathodes, and/or photo-multipliers.
Similarly, in FIG. 3, a side cross-sectional view of a typical imaging optical encoder 300 in combination with an encoded media 302 is shown. The imaging optical encoder 300 may include a read-head 304, where the read-head 304 may include an emitter module 306, and a detector module 308. Similar to both FIGS. 1 and 2, the read-head 304 and the encoded media 302 may move freely relative to each other in either a linear or rotational manner based on whether the reflective optical encoder 300 is either a linear or rotational optical encoder.
The emitter module 306 and the detector module 308 may include optics capable of emitting and detecting, respectively, optical radiation from the emitter module 306 to the detector module 308. The optical radiation may include emitted optical radiation 310, which is emitted by the emitter module 306 on to the encoded media 302, and reflected optical radiation 312, which is reflected to the detector module 308 by the encoded media 302.
It is appreciated by those skilled in the art that the optical radiation again may be visible, infrared, and/or ultraviolet light radiation. The emitting module 306 may include a light source (not shown) such as a diode, a LED, photo-cathode, and/or a light bulb, and the detector module 308 may include an array of photo-detectors (not shown) such as photo-diodes, photo-cathodes, and/or photo-multipliers.
In FIG. 4, a top-view of a typical transmissive or reflective linear encoded media 400 utilized as a codestrip by a linear optical encoder (not shown) is shown. The encoded media 400, FIG. 4, may include an alternating pattern of light regions (i.e., light bars 402) and dark regions (i.e., dark bars 404). Utilizing the encoded media 400, the linear optical encoder may determine the velocity and acceleration of the read-head (not shown) relative to the encoded media 400.
Similarly, in FIG. 5, a top-view of a typical transmissive or reflective rotational encoded media 500 utilized as a codewheel on a wheel shaft 502 by a rotational optical encoder (not shown) is shown. The encoded media 500, FIG. 5, may include an alternating pattern of light regions (i.e., light bars 504) and dark regions (i.e., dark bars 506). Utilizing the encoded media 500, the rotational optical encoder may determine the rotational velocity and acceleration of the read-head (not shown) relative to the encoded media 500.
The light and dark regions in both FIGS. 4 and 5 may contain opaque and transparent segments, respectively, that interrupt the optical radiation from the emitter module to the detector module in the optical detector. In the case of a transmissive optical encoder, the optical radiation directly transmitted from the emitter module to the detector module is interrupted by the encoded media; while in the case of the reflective or imaging optical encoder, the optical radiation from the emitter module is either reflected to the detector module by the encoded media or transmitted through the encoded media away from the detector module.
The optical encoder output is then either a binary “ON” or “OFF,” depending on whether the optical encoder is over a light or dark region on the encoded media in the transmissive optical encoder or whether the optical radiation is reflected on to the detector module. The electronic signals generated by the optical encoder are then passed to a controller that is capable of determining the position and velocity of the optical detector based upon the received signals.
In general, transmissive optical encoders are capable of operating at high speed and high resolution because of their good contrast capabilities. Unfortunately, however, transmissive optical encoders require packaging designs with high profiles because the emitter module and detector module need to be placed opposite each other around the encoder media.
Reflective optical encoders have better packaging designs than transmissive optical encoders because the emitter module and detector module are located substantially on the same plane and may be integrated into a single semiconductor substrate in an integrated circuit. This results in lower profile packaging design with less materials and less assembly complexity than transmissive optical encoders. Unfortunately, typical reflective optical encoders suffer from lower contrast capabilities than transmissive optical encoders and therefore have limits in speed and resolution compared to transmissive optical encoders.
Imaging optical encoders typically have the same benefits as reflective optical encoders in terms of profile, materials and assembly complexity. However, imaging optical encoders require a diffusive encoded media that at present is not maturely established technologically. Additionally, imaging optical encoders suffer from diffusive reflectance and also have limits in speed and resolution compared to transmissive optical encoders.
In FIG. 6, another side cross-sectional view of a typical reflective optical encoder 600 in combination with an encoded media 602 is shown. The reflective optical encoder 600 may include a read-head 604, where the read-head 604 may include an emitter module 606, and a detector module 608. Similar to FIGS. 1, 2, and 3, the read-head 604 and the encoded media 602 may move freely relative to each other in either a linear or rotational manner based on whether the reflective optical encoder 600 is either a linear or rotational optical encoder.
The emitter module 606 and the detector module 608 may include optics capable of emitting and detecting, respectively, optical radiation from the emitter module 606 to the detector module 608. The optical radiation may include emitted optical radiation 610, which is emitted by the emitter module 606 on to the encoded media 602, and reflected optical radiation 612, which is reflected to the detector module 608 by the encoded media 602. Additionally, both emitter module 606 and detector module 608 may be mounted on a common substrate 614. The substrate may be a single semiconductor substrate in an integrated circuit, a lead-frame, an insert-molded lead-frame, a printed circuit board (“PCB”), flexible circuit, ceramics substrate or micro-interconnecting device (“MID”).
It is appreciated that the optical radiation again may be visible, infrared, and/or ultraviolet light radiation. The emitting module 606 may include a light source (not shown) such as a diode, a LED, photo-cathode, and/or a light bulb, and the detector module 608 may include an array of photo-detectors (not shown) such as photo-diodes, photo-cathodes, and/or photo-multipliers.
The optics may include a transmissive layer 616 capable of covering both the emitter module 606 and detector module 608, where the transmissive layer 616 may include any transmissive and moldable material capable of collimating the emitted optical radiation 610 into a parallel beam of optical radiation directed from the emitter module 606 to the encoded media 602. The transmissive layer 616 may be an epoxy layer.
Unfortunately, a problem with this type of reflective optical encoder 600 is that undesired optical radiation 618 from the emitter module 606 is propagated through the epoxy to the detector module 608 causing noise in the detector module 608 that results in loss of image contrast of the light and dark regions (i.e., the “bars”) on the encoded media 602. This loss in image contrast limits the speed and resolution of the typically known reflective optical encoder 600. Therefore, there is a need for an improved reflective optical encoder that is capable of providing higher image contrast and resolution than known reflective optical encoders